This invention relates to discharge excitation pulsed laser oscillation devices (pulsed transversely excited gas laser devices), and more particularly to the electrode structure of the discharge excitation pulsed laser oscillation devices.
FIG. 2 is a schematic sectional view of a conventional discharge excitation pulsed laser oscillation device (pulsed transversely excited gas laser device), showing a section thereof perpendicular to the optical axis of the laser. The device is disclosed in: R. Marchetti and E. Penco, "A new type of corona-discharge photoionization source for gas lasers", Journal of Applied Physics 56(11), December 1984, pp. 3163-3168. In FIG. 2, a pair of main electrodes 1 and 2 opposing each other extend parallel to the optical axis of the laser. Across the main electrodes 1 and 2 is formed a main discharge 3. A laser gas flow 4 is supplied from a laser gas supply device (not shown), along a direction perpendicular to the optical axis of the laser, to the region between the main electrodes 1 and 2.
A pair of preionizers 5 disposed on the upstream and the downstream side (with respect to the direction of the laser gas flow 4) of the main electrodes 1 and 2 preionize the laser gas between the main electrodes 1 and 2 such that the main discharge 3 is generated. Each preionizer 5 consists of: a hollow dielectric pipe 6, an auxiliary electrode 7 inserted into the bore of the dielectric pipe 6, and a conductor wire 8 which is attached over the outer surface of the dielectric pipe 6 and is kept at the same potential as the main electrode 2.
Next, the operation of the discharge excitation pulsed laser oscillation device of FIG. 2 is described. First, the laser gas is supplied from a laser gas supply device to the region between the main electrodes 1 and 2. When a voltage is applied across the main electrode 2 and each of the auxiliary electrodes 7, the corona discharge 9 starts from the point at which the conductor wire 8 and the dielectric pipe 6 are in contact and covers the outer surface of the dielectric pipe 6. The ultraviolet radiation radiated from the corona discharge 9 preionizes the laser gas between the main electrodes 1 and 2. Next, upon application of a voltage across the main electrodes 1 and 2, the preionized laser gas begins to discharge, and the main discharge 3 is generated across the main electrode 1 and main electrode 2. The laser gas is excited by the main discharge 3, such that the laser commences to oscillate in a direction perpendicular to the surface of the drawing in FIG. 2.
In the case of the discharge excitation pulsed laser oscillation device of FIG. 2, the discharge remnants, such as the ions and metastable gas molecules and atoms generated by the main discharge 3, and particles sputtered out from the electrodes, are carried downstream with the laser gas flow 4. By the time when the next main discharge 3 is started by the subsequent voltage pulse, the new laser gas is supplied to the region between the main electrodes 1 and 2 and the ultraviolet radiation is radiated thereto from both sides. Thus, the number of preionized electrons (photoelectrons) remains constant and the distribution thereof is symmetric for the upstream and the downstream side (with respect to the laser gas flow 4) in the region between the main electrodes 1 and 2. Thus, the main discharge 3 is maintained constant throughout the operation.
FIG. 3 is a schematic sectional view of a conventional discharge excitation excimer laser oscillation device. The device is disclosed in: T. S. Fahlen, "HIGH-AVERAGE-POWER EXCIMER LASER", DOE/SF/90024-T2,1977, United States Energy Research and Development Administration. A tube 1a having an outer diameter of 1/4 inches is mounted on an end of a main electrode 1 opposing the other main electrode 2. The preionizer 5, including a hollow dielectric pipe 6 and an auxiliary electrode 7 which consists of a wire inserted into the bore of the dielectric pipe 6, is disposed at the downstream side (with respect to the laser gas flow 4) of the main electrode 1 such that the dielectric pipe 6 is partly in contact with the main electrode 1.
The operation of the laser device of FIG. 3 is similar to that of the laser device of FIG. 2. After the laser gas is supplied to the region between the main electrodes 1 and 2, a voltage is applied across the main electrode 1 and the auxiliary electrode 7, such that the corona discharge 9 is generated from the point where the main electrode 1 and the dielectric pipe 6 are in contact, and thence covers the whole dielectric pipe 6. The laser gas between the main electrodes 1 and 2 is preionized by the ultraviolet radiation radiated from the corona discharge 9. When a voltage is applied across the main electrodes 1 and 2, the preionized laser gas commences to discharge, thereby generating the main discharge 3. The laser gas is excited by the main discharge 3, such that the laser oscillates in the direction perpendicular to the surface of the drawing. The preionizer 5 is disposed only at the downstream side of the main electrode 1. Thus, this excimer laser device has a simplified structure.
The above conventional laser devices have the following disadvantages.
In the case of the device of FIG. 2, the preionizers 5 are disposed on both the upstream and downstream side (with respect to the laser gas flow 4) of the main electrodes 1 and 2. However, the ultraviolet radiation radiated from the preionizer 5 at the downstream side is absorbed by the discharge remnants generated by the main discharge 3. Thus, as the oscillation frequency increases, the center of the distribution of preionized electrons (photoelectrons) is shifted to the upstream side. Thus, as shown in FIG. 4, the central position of the laser beam is also shifted to the upstream side (with respect to the laser gas flow 4) as the oscillation frequency increases.
On the other hand, in the case of the laser device of FIG. 3, the preionizer 5 is disposed only at the downstream side (with respect to the laser gas flow 4) of the main electrodes 1 and 2. The ultraviolet radiation radiated from the preionizer 5 is absorbed by the discharge remnants generated by the main discharge 3. Thus, as the oscillation frequency increases, the number of preionized electrons decreases, and the homogeneity of the main discharge 3 is deteriorated. As a result, the energy is not effectively deposited into the laser gas. Thus the laser oscillation efficiency deteriorates and the output power decreases. Thus, it is necessary to substitute the laser gas between the main electrodes 1 and 2 by a new volume of the laser gas by the time when the next main discharge 3 is generated. To accomplish this, a laser gas supply device of large capacity becomes necessary.